This invention relates to top coat compositions containig microgel particles.
The exterior of automobiles and trucks, for example, are finished with a colored or pigmented coat with or without a clear coat over the color coat for decorative and protective purposes. In either case, the top coat, whether pigmented or not, must exhibit an excellent durability and appearance. As will be apparent, since the quality of the top coat directly affects the appearance of the entire finish, it has been a problem to improve the appearance of the top coat as much as possible.
One approach for solving this problem would be to make the top coat as thick as possible. However, with the conventional top coat compositions, the thickness of the finished films is limited to only 30-35 microns with a single coating operation, or 40-45 microns with two coating operations. This is because the conventional coating compositions tend to excessively run with an increase in the amount applied per unit area. Thick top coats may be provided by multiple coating operations but this greatly lowers productivity and requires an extensive modification of existing production lines.
In order to avoid undersirable run, very fine, generally spherical particles of a cross-linked polymer, generally known as microgel particles, may be incorporated onto a system comprising a film-forming polymer, a cross-linking agent and a liquid diluent. The system is imparted with a structural viscosity by the addition of the microgel particles and thus may be applied to a greater film thickness in a single application than is possible with the conventional system without appreciable run. See, Japanese Laid Open Patent Applications 49-97026 and 58-129065.
Experiments have shown, however, that systems containing the microgel particles do not necessarily improve the appearance of the top coat when applied to a greater film thickness and often tend to decrease the quality of the finish in terms of gloss or luster. This is particularly the case when the system is pigmented and applied to side portions of automobiles such as doors where the applied films are subjected to a shear force by gravity.
In order that a coating system containing microgel particles can exhibit its characteristics to the fullest extent, it is essential for the particles to be insoluble in their dispersing medium (consisting essentially of a film-forming polymer, a cross-linking agent thereof and an organic solvent). Thus the particles remain dispersed as such throughout the entire top coat after curing. In case of clear top coats, if the difference in refraction index between the microgel particles and their surrounding matrix is too great, irregular or random reflection will take place at interfaces therebetween. If such irregular reflection takes place, the top coat will look white and opaque thereby injuring the appearance of the entire finish.
Similarly, irregular reflection can take place in colored or pigmented top coats owing to a difference in refraction index. This is because a very thin clear layer free from pigment particles is formed adjacent the surface. According to our experiments, when the exterior of automobiles was coated with a conventional pigmented top coat system containing microgel particles to a fully cured film thickness which is about 20 microns greater than conventional, areas having poor gloss were observed on side portions such as doors.
It is for this reason that the previously cited Japanese Laid Open Patent Application 49-97026 suggests the combination of microgel particles and a film-forming polymer having substantially the same refraction index. The microgel particles used therein are produced by the non-aqueous dispersion (NAD) method wherein a mixture of ethylenically unsaturated comonomers including a cross-linking comonomer is polymerized in a non-aqueous organic solvent such as aliphatic hydrocarbon solvents in which the comonomers are soluble but the resulting polymer is insoluble in the presence of a stabilizing agent. This necessarily limits the selection of usable comonomers only to those having solubility parameters which are relatively different from those of the non-aqueous solvents.
Said Japanese Application 49-97026 further requires the refraction index of microgel particles to be substantially same as that of the main film-forming polymer. However, since it is not the film-forming polymer per se but its cross-linked products which surround the microgel particles in the finished state, the problem of poor appearance owing to irregular reflection to light by the top coat has not been fully overcome by this technique.
Accordingly, it is a principal object of the invention to provide a coating system containing microgel particles which may obviate the foregoing defects of known technique and which permits wide selection of microgel particles while upgrading the resulting finish when used as a top finish. Other objects and advantages of the present invention will become apparent as the description proceeds.